In order to measure atmospheric pollutants which may be present in air at low concentrations e.g. less than 5 or 10 ppb, it is necessary to provide air monitoring instruments which are sensitive, specific and which perform reliably for a long period of time.
One way to provide specificity is to provide a filter between an inlet into electrochemical gas sensing apparatus and the sensing electrode, where the filter is selected to remove one or more gas species which may otherwise interfere with measurements of the target analyte gas.
It has been proposed to use MnO2 as a filter material in a conductometric sensor to filter O3 out of ambient air (Viricelle J P et al., Materials Science and Engineering C, Elsevier Science S. A., CH, vol. 26, no. 2-3, pages 186-195). EP 2975390 (Alphasense Limited) proposed the use of an MnO2 filter adjacent one of two carbon sensing electrodes which receive gas in parallel, in a sensor for the detection of NO2 and/or O3, without interference from SO2, CO, NO, NH3 and H2.
Although MnO2 is useful as a filter material to filter O3 out of ambient air, for example in a sensor for the detection of NO2 and/or O3, it can create a problem with cross-sensitivity to NO. Without wanting to be bound by theory, we hypothesize that NO reacts with MnO2 to form NO2 which then reacts at the sensing electrode.
Accordingly, the invention seeks to provide an improved sensor for the detection of at least one gaseous analyte. Some embodiments relate to the detection of NO2 and/or O3.